Antiferromagnetic Spintronics Research Articles

Route towards stable homochiral topological textures in A -type antiferromagnets

Topologically protected whirling magnetic textures could emerge as data carriers in next-generation post-Moore computing. Such textures are abundantly observed in ferromagnets (FMs); however, their antiferromagnetic (AFM) counterparts are expected to be even more relevant for device applications, as they promise ultra-fast, deflection-free dynamics whilst being robust against external fields. Unfortunately, they have remained elusive, hence identifying materials hosting such textures is key to developing this technology. Here, we present comprehensive micromagnetic and analytical models investigating topological textures in the broad material class of A-type antiferromagnets, specifically focusing on the prototypical case of $\alpha \text{-Fe}_2 \text{O}_3$,an emerging candidate for AFM spintronics. By exploiting a symmetry breaking interfacial Dzyaloshinskii-Moriya interaction (iDMI), it is possible to stabilize a wide topological family, including AFM (anti)merons and bimerons and the hitherto undiscovered AFM skyrmions. Whilst iDMI enforces homochirality and improves the stability of these textures, the widely tunable anisotropy and exchange interactions enable unprecedented control of their core dimensions. We then present a unifying framework to model the scaling of texture sizes based on a simple dimensional analysis. As the parameters required to host and tune homochiral AFM textures may be obtained by rational materials design of $\alpha \text{-Fe}_2 \text{O}_3$, it could emerge as a promising platform to initiate AFM topological spintronics.

Antiferromagnetic skyrmion based shape-configured leaky-integrate-fire neuron device

Spintronic devices based on antiferromagnetic (AFM) skyrmion motion on the nanotracks have gained significant interest as a key component of neuromorphic data processing systems. AFM skyrmions are favorable over the ferromagnetic (FM) skyrmions as they follow the straight trajectories and prevent its annihilation at the nanotrack edges. In this paper, the AFM skyrmion-based neuron device that exhibits the leaky-integrate-fire functionality is proposed for the first time. It exploits the current-driven skyrmion dynamics on the shape-configured nanotracks that are linearly decreasing and exponentially decaying. The device structure creates the regions from lower to higher energy states for the AFM skyrmions during its motion from the wider to narrower region. This causes the repulsion force from the nanotrack edges to act on the AFM skyrmion thereby, drifting it in the backward direction in order to minimize the system energy. This provides the leaking functionality to the neuron device without any external stimuli and additional hardware cost. The average velocities during the integration and leaky processes are in the order of 103 and 102 m s−1, respectively, for the linearly and exponentially tapered nanotracks. Moreover, the energy of the skyrmion is in the order 10−20 J. Hence, the suggested device opens up the path for the development of high-speed and energy-efficient devices in AFM spintronics for neuromorphic computing.

Chemical Potential Switching of the Anomalous Hall Effect in an Ultrathin Noncollinear Antiferromagnetic Metal.

The discovery of the anomalous Hall effect in noncollinear antiferromagnetic metals represents one of the most important breakthroughs for the emergent antiferromagnetic spintronics. The tuning of chemical potential has been an important theoretical approach to varying the anomalous Hall conductivity, but the direct experimental demonstration has been challenging owing to the large carrier density of metals. In this work, an ultrathin noncollinear antiferromagnetic Mn3 Ge film is fabricated and its carrier density is modulated by ionic liquid gating. Via a small voltage of ≈3V, its carrier density is altered by ≈90% and, accordingly, the anomalous Hall effect is completely switched off. This work thus creates an attractive new way to steering the anomalous Hall effect in noncollinear antiferromagnets.

Tunneling Magnetoresistance in Noncollinear Antiferromagnetic Tunnel Junctions.

Antiferromagnetic (AFM) spintronics has emerged as a subfield of spintronics driven by the advantages of antiferromagnets producing no stray fields and exhibiting ultrafast magnetization dynamics. The efficient method to detect an AFM order parameter, known as the Néel vector, by electric means is critical to realize concepts of AFM spintronics. Here, we demonstrate that noncollinear AFM metals, such as Mn_{3}Sn, exhibit a momentum dependent spin polarization which can be exploited in AFM tunnel junctions to detect the Néel vector. Using first-principles calculations, we predict a tunneling magnetoresistance (TMR) effect as high as 300% in AFM tunnel junctions with Mn_{3}Sn electrodes, where the junction resistance depends on the relative orientation of their Néel vectors and exhibits four nonvolatile resistance states. We argue that the spin-split band structure and the related TMR effect can also be realized in other noncollinear AFM metals like Mn_{3}Ge, Mn_{3}Ga, Mn_{3}Pt, and Mn_{3}GaN. Our work provides a robust method for detecting the Néel vector in noncollinear antiferromagnets via the TMR effect, which may be useful for their application in AFM spintronic devices.

Photonic-Crafting of Non-Volatile and Rewritable Antiferromagnetic Spin Textures with Drastic Difference in Electrical Conductivity.

Antiferromagnetic spintronics is an emerging field of non-volatile data storage and information processing. The zero net magnetization and zero stray fields of antiferromagnetic materials eliminate interference between neighbor units, leading to high-density memory integrations. However, this invisible magnetic character at the same time also poses a great challenge in controlling and detecting magnetic states in antiferromagnets. Here, two antiferromagnetic spin states close in energy in strained BiFeO3 thin films at room temperature are discovered. It can be reversibly switched between these two non-volatile antiferromagnetic states by a moderate magnetic field and a non-contact optical approach. Importantly, the conductivity of the areas with each antiferromagnetic textures is drastically different. It is conclusively demonstrated the capability of optical writing and electrical reading of these newly discovered bistable antiferromagnetic states in the BiFeO3 thin films.

Strain-dependent magnetism and anomalous Hall effect in noncollinear antiferromagnetic Mn3Pt films

The exploration of Berry curvature physics and the related non-trivial magnetic transport of two-dimensional kagome spin-lattice structure of Mn atoms in noncollinear antiferromagnetic materials, has received widespread attention in recent years. Current research is mainly focused on the hexagonal chiral antiferromagnets such as Mn3Sn. However, there are few researches about face-centered cubic (fcc) noncollinear antiferromagnetism. In this work, the influence of strain on the magnetic properties and the anomalous Hall effect (AHE) of the fcc noncollinear antiferromagnetic Mn3Pt films were systematically explored. The results showed that the ferromagnetic signal derived from the tilt of the magnetic moment of Mn atoms in the kagome structures, as well as the anomalous Hall resistance originating from the non-zero Berry curvature, had comparable response tendency to the strain, differing from hexagonal chiral antiferromagnets. An anomalous Hall conductivity exceeding 100 (Ω cm)−1 was obtained by tuning the film thickness of Mn3Pt. In addition, the relationship among the Berry phase, magnetic properties, and AHE in Mn3Pt was further verified by regulating the crystal growth orientation. The results established Mn3Pt as a promising candidate for its integration with room-temperature antiferromagnetic spintronics.

Exchange-Torque-Triggered Fast Switching of Antiferromagnetic Domains.

The antiferromagnet is considered to be a promising hosting material for the next generation of magnetic storage due to its high stability and stray-field-free property. Understanding the switching properties of the antiferromagnetic (AFM) domain state is critical for developing AFM spintronics. By utilizing the magneto-optical birefringence effect, we experimentally demonstrate the switching rate of the AFM domain can be enhanced by more than 2 orders of magnitude through applying an alternating square-wave field on a single crystalline Fe/CoO bilayer. The observed extraordinary speed can be much faster than that triggered by a constant field with the same amplitude. The effect can be understood as the efficient suppression of the pinning of AFM domain walls by the strong exchange torque triggered by the reversal of the Fe magnetization, as revealed by spin dynamics simulations. Our finding opens up new opportunities to design the antiferromagnet-based spintronic devices utilizing the ferromagnet-antiferromagnet heterostructure.

Tunable Spin Hall Magnetoresistance in All-Antiferromagnetic Heterostructures

We investigate the spin Hall magnetoresistance (SMR) in all-antiferromagnetic heterostructures α-Fe2O3/Cr2O3 with Pt contacts. When the temperature is ultralow (< 50 K), the spin current generated in the Pt layer cannot be transmitted through Cr2O3 (t = 4 nm), and the SMR is near zero. Meanwhile, when the temperature is higher than the spin fluctuation temperature T F (≈ 50 K) of Cr2O3 and lower than its Néel temperature T N (≈ 300 K), the spin current goes through the Cr2O3 layer and is reflected at the α-Fe2O3/Cr2O3 interface; an antiferromagnetic (negative) SMR is observed. As temperature increases higher than T N, paramagnetic (positive) SMR mainly arises from the spin current reflection at the Cr2O3/Pt interface. The transition temperatures from negative to positive SMR are enhanced with increasing Cr2O3 layer thickness, accompanied by the absence of SMR signals when t = 10 nm. Such a tunable SMR builds a bridge between spin transport and structures. It also enriches antiferromagnetic spintronics.

Electrical switching of antiferromagnetic CoO | Pt across the Néel temperature

One of the most important challenges in antiferromagnetic spintronics is the readout of the Néel vector state. High current densities up to 108 Acm−2 used in the electrical switching experiments cause notorious difficulty in distinguishing between magnetic and thermal origins of the electrical signals. To overcome this problem, we present a temperature dependence study of the transverse resistance changes in the switching experiment with CoO | Pt devices. We demonstrate the possibility to extract a pattern of spin Hall magnetoresistance for current pulses with a density of 5×107A cm−2 that is present only below the Néel temperature and does not follow a trend expected for thermal effects. This is the compelling evidence for the magnetic origin of the signal, which is observed using purely electrical techniques. We confirm these findings by complementary experiments in an external magnetic field. Such an approach can allow determining the optimal conditions for switching antiferromagnets and be very valuable when no imaging techniques can be applied to verify the origin of the electrical signal.

Magnetic Configuration Driven Femtosecond Spin Dynamics in Synthetic Antiferromagnets.

Ultrafast demagnetization in diverse materials has sparked immense research activities due to its captivating richness and contested underlying mechanisms. Among these, the two most celebrated mechanisms have been the spin-flip scattering (SFS) and spin transport (ST) of optically excited carriers. In this work, we have investigated femtosecond laser-induced ultrafast demagnetization in perpendicular magnetic anisotropy-based synthetic antiferromagnets (p-SAFs) where [Co/Pt]n-1/Co multilayer blocks are separated by Ru or Ir spacers. Our investigation conclusively shows that the ST of optically excited carriers can have a significant contribution to the ultrafast demagnetization in addition to SFS processes. Moreover, we have also achieved an active control over the individual mechanisms by specially designing the SAF samples and altering the external magnetic field and excitation fluence. Our study provides a vital understanding of the underlying mechanism of ultrafast demagnetization in synthetic antiferromagnets, which will be crucial in future research and applications of antiferromagnetic spintronics.

Enhancement of current to spin-current conversion and spin torque efficiencies in a synthetic antiferromagnetic layer based on a Pt/Ir/Pt spacer layer

We investigated the current-induced spin-orbit torque (SOT) originating from the spin Hall effect in stack systems with perpendicularly magnetized Co/Pt/Ir/Pt/Co synthetic antiferromagnetic (AFM) structures which exhibit nearly compensated magnetization and interlayer exchange coupling of ${J}_{\mathrm{ex}}=0.11\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/{\mathrm{m}}^{2}$. The results were compared with ferromagnetic stack systems with perpendicularly magnetized Pt/Co and (Ir/Pt)-multilayer/Co structures. The magnetizations of the two Co layers in the Co/Pt/Ir/Pt/Co synthetic AFMs can be switched between two antiparallel states simultaneously by SOT. By this switching mechanism, the current to spin-current conversion and spin-torque efficiencies in a synthetic AFM layer stack based on a Pt/Ir/Pt spacer layer are about twice as high as those of a ferromagnetic stack using a Pt heavy metal electrode. The efficient switching of compensated synthetic AFMs would advance magnetic memory devices with high density, high speed, and low power consumption. We expect the Pt/Ir/Pt spacer layer paves the way to AFM spintronics based on multilayer systems.

Electrical control of ON–OFF magnetism and exchange bias via reversible ionic motion

The lithium-based magneto-ionic effect is an emerging and promising way to control magnetism. Here, a lithium-ion battery-structure is adopted to control the magnetic properties of the Co/CoO bilayer. Using operando magnetometry, we performed a reversible toggling of lithium ion induced magnetic phase transition between nano-Co (superparamagnetism) and CoO (antiferromagnetism), which enables an ON–OFF magnetism switching. On this basis, a robust modulation of exchange bias by electric field at low temperature is achieved. In addition, reversible tuning of coercivity and magnetization up to 71% and 118%, respectively, in a Co/CoO bilayer at room temperature are performed. This work provides a more efficient means to modulate antiferromagnetism and exchange bias, facilitating the development of antiferromagnetic spintronics.

Evidence of magnetism-induced topological protection in the axion insulator candidate EuSn2P2

We unravel the interplay of topological properties and the layered (anti)ferromagnetic ordering in EuSn2P2, using spin and chemical selective electron and X-ray spectroscopies supported by first-principle calculations. We reveal the presence of in-plane long-range ferromagnetic order triggering topological invariants and resulting in the multiple protection of topological Dirac states. We provide clear evidence that layer-dependent spin-momentum locking coexists with ferromagnetism in this material, a cohabitation that promotes EuSn2P2 as a prime candidate axion insulator for topological antiferromagnetic spintronics applications.

Antiferromagnetic Magnon Drag Effect and Giant On–Off Ratio in a Vertical Device

AbstractAntiferromagnets offer unique advantages in the development of spintronics because of the absence of stray field, insensitivity to external disturbances and the ultrafast terahertz eigenfrequency, advancing the development of devices toward small‐size and high‐speed. However, the ultralow read‐out signals of antiferromagnetic spintronics are notoriously smaller than their ferromagnetic counterpart, seriously hindering the development of antiferromagnets. Thus, experimental realization of large read‐out signals in antiferromagnets, especially in vertical devices, is intensely pursued. Here the antiferromagnetic magnon drag effect and a giant on–off ratio of ≈1000 are realized in a vertical sandwich structure of heavy metal/antiferromagnetic insulator/heavy metal (Pt/α‐Fe2O3/Pt). The read‐out signal can exist up to room temperature, and the detected signal owing to the transmission and blocking of magnon strongly relies on the relative orientation between spin polarization and Néel vector. Benefiting from the nonvolatile characteristic of Fe2O3, zero‐field vertical magnon transport is achieved. The giant on–off signals in vertical devices that can be tightly packed, can greatly promote the practical application of antiferromagnetic magnon devices toward smaller, faster, and lower energy consumption.

Extraordinary anisotropic magnetoresistance in CaMnO3/CaIrO3 heterostructures

The realization of fourfold anisotropic magnetoresistance (AMR) in $3d\ensuremath{-}5d$ heterostructures has boosted major efforts in antiferromagnetic (AFM) spintronics. However, despite the potential of incorporating strong spin-orbit coupling, only small AMR signals have been detected thus far, prompting a search for mechanisms to enhance the signal. In this paper, we demonstrate an extraordinarily elevated fourfold AMR of 70% realized in $\mathrm{CaMn}{\mathrm{O}}_{3}/\mathrm{CaIr}{\mathrm{O}}_{3}$ thin film superlattices. We find that the biaxial magnetic anisotropy and the spin-flop transition in a nearly Mott insulating phase form a potent combination, each contributing one order of magnitude to the total signal. Dynamics between these phenomena capture a subtle interaction of pseudospin coupling with the lattice and external magnetic field, an emergent phenomenon creating opportunities to harness its potential in AFM spintronics.

Noncollinear Antiferromagnetic Spintronics

Antiferromagnetic spintronics is one of the leading candidates for next-generation electronics. Among abundant antiferromagnets, noncollinear antiferromagnets are promising for achieving practical applications due to coexisting ferromagnetic and antiferromagnetic merits. In this perspective, we briefly review the recent progress in the emerging noncollinear antiferromagnetic spintronics from fundamental physics to device applications. Current challenges and future research directions for this field are also discussed.

Manipulation of Time- and Frequency-Domain Dynamics by Magnon-Magnon Coupling in Synthetic Antiferromagnets

Magnons (the quanta of spin waves) could be used to encode information in beyond Moore computing applications. In this study, the magnon coupling between acoustic mode and optic mode in synthetic antiferromagnets (SAFs) is investigated by micromagnetic simulations. For a symmetrical SAF system, the time-evolution magnetizations of the two ferromagnetic layers oscillate in-phase at the acoustic mode and out-of-phase at the optic mode, showing an obvious crossing point in their antiferromagnetic resonance spectra. However, the symmetry breaking in an asymmetrical SAF system by the thickness difference, can induce an anti-crossing gap between the two frequency branches of resonance modes and thereby a strong magnon-magnon coupling appears between the resonance modes. The magnon coupling induced a hybridized resonance mode and its phase difference varies with the coupling strength. The maximum coupling occurs at the bias magnetic field at which the two ferromagnetic layers oscillate with a 90° phase difference. Besides, we show how the resonance modes in SAFs change from the in-phase state to the out-of-phase state by slightly tuning the magnon-magnon coupling strength. Our work provides a clear physical picture for the understanding of magnon-magnon coupling in a SAF system and may provide an opportunity to handle the magnon interaction in synthetic antiferromagnetic spintronics.

Intrinsic Nonlinear Hall Effect in Antiferromagnetic Tetragonal CuMnAs.

Detecting the orientation of the Néel vector is a major research topic in antiferromagnetic spintronics. Here we recognize the intrinsic nonlinear Hall effect, which is independent of the relaxation time, as a prominent contribution to the time-reversal-odd second order conductivity and can be used to detect the reversal of the Néel vector. In contrast, the Berry-curvature-dipole-induced nonlinear Hall effect depends linearly on relaxation time and is time-reversal even. We study the intrinsic nonlinear Hall effect in an antiferromagnetic metal: tetragonal CuMnAs, and show that its nonlinear Hall conductivity can reach the order of mA/V^{2}. The dependence on the chemical potential of such nonlinear Hall conductivity can be qualitatively explained by a tilted massive Dirac model. Moreover, we demonstrate its strong temperature dependence and briefly discuss its competition with the second order Drude conductivity. Finally, a complete survey of magnetic point groups is presented, providing guidelines for finding more antiferromagnetic materials with the intrinsic nonlinear Hall effect.

Spin gapless semiconductors in antiferromagnetic monolayer HC[formula omitted]N[formula omitted]BN under strain

A density functional study is presented for HC4N3BN monolayer, which is triazine g-C4N3 tailored with H at its graphitic sites and B plus N at vacant sites, under uniaxial or biaxial in-plane strain. For moderate strains up to about ±8 percent, first-principles molecular dynamics simulations prove its thermal stability at room temperature and electronic structure calculations show the persistence of antiferromagnetic ground state of the system. The material undergoes magnetic transitions to ferro- and ferrimagnetic orders respectively at the critical biaxial strain of −8.3% and uniaxial strain of 7.8%. Interestingly, both transitions occur when the electronic structures of strained systems show the character of a spin gapless semiconductor. The combined effect of spin charge transfer and band shifting is proposed as a possible explanation for these transitions. Our finding on this monolayer signifies the importance of strain engineering in designing novel materials for antiferromagnetic spintronics.

Colossal angular magnetoresistance in the antiferromagnetic semiconductor EuTe2

With a surge of interest in antiferromagnetic (AFM) spintronics, the manipulation and detection of different AFM states has become a topic of concern. In particular, a large magnetoresistive response is of critical importance for facilitating the readout of AFM states. Here, we report the observation of colossal angular magnetoresistance (AMR) in a layered AFM rare-earth dichalcogenide, ${\mathrm{EuTe}}_{2}$. The AMR reaches \ensuremath{\sim}40 000% at 2 K and 22 kOe, which is approximately four orders of magnitude larger than in AFM metals and alloys. Combined magnetization, resistivity, and theoretical analysis reveal strong coupling of the electronic state (metal vs insulator) to the vector spins, which mainly arises from the space-time inversion symmetry-broken correlated band reconstruction. Consequently, the anisotropic magnetization response to the vector magnetic field induces metal-insulator transition (MIT) and a large anisotropic magnetotransport that explains the experimental findings very well. These results suggest an avenue for exploring the spin configuration dependent transport properties for unique AFM spintronic devices.